This invention relates to television broadcasting and, more particularly, to an apparatus and method for processing television video signals in a broadcasting plant.
In a television broadcasting plant there are generally a plurality of video signals generated and available for use in forming the ultimately transmitted program video. The video signals may be available from different television cameras or video tape machines and are typically combined in various ways to achieve desired sequences and special effects in the program video. In order to achieve the orderly combination of different encoded video signals, it is necessary that they be time synchronized, and this is readily accomplished by using a common source of synchronizing signals, denoted as "plant sync," when encoding each video signal.
A problem arises, however, when an already encoded video signal is subjected to processing whereby it is unavoidably time delayed. Unless all other encoded video signals are also subjected to the same time delay, limitations arise in utilizing the processed video signal for special effects in conjunction with other video signals. For example, to perform a chroma keying operation on a pair of already encoded video signals it is necessary to decode at least one of the signals in order to derive the appropriate keying signal, such as a keying signal which is active during blue picture areas. The decoding operation typically introduces a delay of, say, 460 ns which, if uncorrected for, would cause a phase error in the color picture signal. Accordingly, by employing a variable delay, the entire video signal output of the chroma keyer system is delayed by a total of 558 ns. In this manner, and since 558 ns is the period of time of two cycles of color subcarrier, the delay introduced by the chroma keyer does not interfere with ultimate decoding of the output of the chroma keyer since the appropriate color phase relationship of the modulated color subcarrier signal is maintained. In fact, the output of the chroma keying apparatus can be used for most conventional television broadcast plant operations since the delay of 558 ns is not noticeable when, for example, this signal is cut into an undelayed video signal, the time difference not being noticeable to the eye. However, when it is desired to utilize certain special effects, such as a "fade" between two video signals, this effect cannot be used since the timing differences between the signals will be noticeable when the signals are effectively "mixed" during the special effects operation. As a consequence, a video signal that has been processed through a chroma keyer, or similar equipment introducing delay to an already encoded video signal, is not utilized for certain types of special effects in the prior art broadcasting plant.
It is one object of the present invention to provide solution to the problem as set forth.